Refactor: Attributes: Add assign_data function

Prevent an extra copy that could happen when calling
data_for_write() to retrieve mutable access, only to just
replace the data.

Pull Request: https://projects.blender.org/blender/blender/pulls/140946

source/blender/blenkernel/BKE_attribute_storage.hh

@@ -108,6 +108,9 @@ class Attribute {
    * \warning Does not yet support attributes stored as a single value (#AttrStorageType::Single).
    */
   DataVariant &data_for_write();
+
+  /** Replace the attribute's data without first making the existing data mutable. */
+  void assign_data(DataVariant &&data);
 };
 
 class AttributeStorageRuntime {
@@ -228,6 +231,11 @@ inline const Attribute::DataVariant &Attribute::data() const
   return data_;
 }
 
+inline void Attribute::assign_data(DataVariant &&data)
+{
+  data_ = std::move(data);
+}
+
 }  // namespace blender::bke
 
 inline blender::bke::AttributeStorage &AttributeStorage::wrap()

source/blender/blenkernel/intern/pointcloud.cc

@@ -232,7 +232,7 @@ static MutableSpan<T> get_mutable_attribute(PointCloud &pointcloud,
       if (const auto *single_data = std::get_if<bke::Attribute::SingleData>(&attr->data())) {
         /* Convert single value storage to array storage. */
         const GPointer g_value(CPPType::get<T>(), single_data->value);
-        attr->data_for_write() = bke::Attribute::ArrayData::ForValue(g_value, pointcloud.totpoint);
+        attr->assign_data(bke::Attribute::ArrayData::ForValue(g_value, pointcloud.totpoint));
       }
       auto &array_data = std::get<bke::Attribute::ArrayData>(attr->data_for_write());
       BLI_assert(array_data.size == pointcloud.totpoint);

source/blender/geometry/intern/randomize.cc

@@ -98,7 +98,7 @@ static void reorder_attribute_domain(bke::AttributeStorage &data,
         bke::attribute_math::gather(GSpan(type, data.data, data.size),
                                     new_by_old_map,
                                     GMutableSpan(type, new_data.data, new_data.size));
-        attr.data_for_write() = std::move(new_data);
+        attr.assign_data(std::move(new_data));
       }
       case bke::AttrStorageType::Single: {
         return;